As it has already been mentioned, in semi-central collisions a strong \(v_2\)  signal is observed.
At beam energies of a few  \(\,A\)  G\(e\) V particle emission in the reaction plane is partially suppressed due to absorption of particles emitted from the collision zone (participant matter) in the bypassing projectile and target (spectator) matter.
The latter effect results in a negative \(v_2\) .
With increasing beam energy the \(v_2\)  signal turns positive (to in-plane) with increasing strength.
At the highest RHIC energies it has been found that \(v_2\)  [1]} for identified baryons and mesons can be collapsed to a unique curve if \(v_2\)  is scaled to the number of constituents quarks \(n_q\)  of the identified particle (i.e. \(v_q = 2\)  for mesons and \(v_q = 3\)  for baryons) and plotted in bins of transverse kinetic energy per constituent quark \(E_T/n_q = \left(m_T - m_0\right)/n_q\) , where \(m_T\)  is the transverse mass and \(m_0\)  the rest mass of the identified hadron.
This observation is commonly called quark number scaling.
The success of the hydrodynamical description of the fireball evolution at ultra-relativistic collision energies supports the assumption of a very rapid thermalization in the initial stage.
In particular the strong flow effects and quark number scaling give very clear evidence for a deconfined phase (partonic) phase of the matter which is strongly coupled.
Moreover, the flow develops throughout the whole expansion until the system freezes out.
Hence, it is to be expected that any penetrating probe, as dileptons, which on average decouple from the system early on, should not follow this flow systematics.
